[PATCH v9 2/8] KVM: Extend the memslot to support fd-based private memory

Chao Peng
2022-10-25T15:19:23Z

In memory encryption usage, guest memory may be encrypted with special
key and can be accessed only by the guest itself. We call such memory
private memory. It's valueless and sometimes can cause problem to allow
userspace to access guest private memory. This new KVM memslot extension
allows guest private memory being provided though a restrictedmem
backed file descriptor(fd) and userspace is restricted to access the
bookmarked memory in the fd.

This new extension, indicated by the new flag KVM_MEM_PRIVATE, adds two
additional KVM memslot fields restricted_fd/restricted_offset to allow
userspace to instruct KVM to provide guest memory through restricted_fd.
'guest_phys_addr' is mapped at the restricted_offset of restricted_fd
and the size is 'memory_size'.

The extended memslot can still have the userspace_addr(hva). When use, a
single memslot can maintain both private memory through restricted_fd
and shared memory through userspace_addr. Whether the private or shared
part is visible to guest is maintained by other KVM code.

A restrictedmem_notifier field is also added to the memslot structure to
allow the restricted_fd's backing store to notify KVM the memory change,
KVM then can invalidate its page table entries.

Together with the change, a new config HAVE_KVM_RESTRICTED_MEM is added
and right now it is selected on X86_64 only. A KVM_CAP_PRIVATE_MEM is
also introduced to indicate KVM support for KVM_MEM_PRIVATE.

To make code maintenance easy, internally we use a binary compatible
alias struct kvm_user_mem_region to handle both the normal and the
'_ext' variants.

Co-developed-by: Yu Zhang <yu.c.zhang@linux.intel.com>
Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
---
Documentation/virt/kvm/api.rst | 48 ++++++++++++++++++++++++++++-----
arch/x86/kvm/Kconfig | 2 ++
arch/x86/kvm/x86.c | 2 +-
include/linux/kvm_host.h | 13 +++++++--
include/uapi/linux/kvm.h | 29 ++++++++++++++++++++
virt/kvm/Kconfig | 3 +++
virt/kvm/kvm_main.c | 49 ++++++++++++++++++++++++++++------
7 files changed, 128 insertions(+), 18 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index eee9f857a986..f3fa75649a78 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -1319,7 +1319,7 @@ yet and must be cleared on entry.
:Capability: KVM_CAP_USER_MEMORY
:Architectures: all
:Type: vm ioctl
-:Parameters: struct kvm_userspace_memory_region (in)
+:Parameters: struct kvm_userspace_memory_region(_ext) (in)
:Returns: 0 on success, -1 on error

::
@@ -1332,9 +1332,18 @@ yet and must be cleared on entry.
__u64 userspace_addr; /* start of the userspace allocated memory */
};

+ struct kvm_userspace_memory_region_ext {
+ struct kvm_userspace_memory_region region;
+ __u64 restricted_offset;
+ __u32 restricted_fd;
+ __u32 pad1;
+ __u64 pad2[14];
+ };
+
/* for kvm_memory_region::flags */
#define KVM_MEM_LOG_DIRTY_PAGES (1UL << 0)
#define KVM_MEM_READONLY (1UL << 1)
+ #define KVM_MEM_PRIVATE (1UL << 2)

This ioctl allows the user to create, modify or delete a guest physical
memory slot. Bits 0-15 of "slot" specify the slot id and this value
@@ -1365,12 +1374,27 @@ It is recommended that the lower 21 bits of guest_phys_addr and userspace_addr
be identical. This allows large pages in the guest to be backed by large
pages in the host.

-The flags field supports two flags: KVM_MEM_LOG_DIRTY_PAGES and
-KVM_MEM_READONLY. The former can be set to instruct KVM to keep track of
-writes to memory within the slot. See KVM_GET_DIRTY_LOG ioctl to know how to
-use it. The latter can be set, if KVM_CAP_READONLY_MEM capability allows it,
-to make a new slot read-only. In this case, writes to this memory will be
-posted to userspace as KVM_EXIT_MMIO exits.
+kvm_userspace_memory_region_ext struct includes all fields of
+kvm_userspace_memory_region struct, while also adds additional fields for some
+other features. See below description of flags field for more information.
+It's recommended to use kvm_userspace_memory_region_ext in new userspace code.
+
+The flags field supports following flags:
+
+- KVM_MEM_LOG_DIRTY_PAGES to instruct KVM to keep track of writes to memory
+ within the slot. For more details, see KVM_GET_DIRTY_LOG ioctl.
+
+- KVM_MEM_READONLY, if KVM_CAP_READONLY_MEM allows, to make a new slot
+ read-only. In this case, writes to this memory will be posted to userspace as
+ KVM_EXIT_MMIO exits.
+
+- KVM_MEM_PRIVATE, if KVM_CAP_PRIVATE_MEM allows, to indicate a new slot has
+ private memory backed by a file descriptor(fd) and userspace access to the
+ fd may be restricted. Userspace should use restricted_fd/restricted_offset in
+ kvm_userspace_memory_region_ext to instruct KVM to provide private memory
+ to guest. Userspace should guarantee not to map the same pfn indicated by
+ restricted_fd/restricted_offset to different gfns with multiple memslots.
+ Failed to do this may result undefined behavior.

When the KVM_CAP_SYNC_MMU capability is available, changes in the backing of
the memory region are automatically reflected into the guest. For example, an
@@ -8215,6 +8239,16 @@ structure.
When getting the Modified Change Topology Report value, the attr->addr
must point to a byte where the value will be stored or retrieved from.

+8.36 KVM_CAP_PRIVATE_MEM
+------------------------
+
+:Architectures: x86
+
+This capability indicates that private memory is supported and userspace can
+set KVM_MEM_PRIVATE flag for KVM_SET_USER_MEMORY_REGION ioctl. See
+KVM_SET_USER_MEMORY_REGION for details on the usage of KVM_MEM_PRIVATE and
+kvm_userspace_memory_region_ext fields.
+
9. Known KVM API problems
=========================

diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
index 67be7f217e37..8d2bd455c0cd 100644
--- a/arch/x86/kvm/Kconfig
+++ b/arch/x86/kvm/Kconfig
@@ -49,6 +49,8 @@ config KVM
select SRCU
select INTERVAL_TREE
select HAVE_KVM_PM_NOTIFIER if PM
+ select HAVE_KVM_RESTRICTED_MEM if X86_64
+ select RESTRICTEDMEM if HAVE_KVM_RESTRICTED_MEM
help
Support hosting fully virtualized guest machines using hardware
virtualization extensions. You will need a fairly recent
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4bd5f8a751de..02ad31f46dd7 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -12425,7 +12425,7 @@ void __user * __x86_set_memory_region(struct kvm *kvm, int id, gpa_t gpa,
}

for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
- struct kvm_userspace_memory_region m;
+ struct kvm_user_mem_region m;

m.slot = id | (i << 16);
m.flags = 0;
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 32f259fa5801..739a7562a1f3 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -44,6 +44,7 @@

#include <asm/kvm_host.h>
#include <linux/kvm_dirty_ring.h>
+#include <linux/restrictedmem.h>

#ifndef KVM_MAX_VCPU_IDS
#define KVM_MAX_VCPU_IDS KVM_MAX_VCPUS
@@ -575,8 +576,16 @@ struct kvm_memory_slot {
u32 flags;
short id;
u16 as_id;
+ struct file *restricted_file;
+ loff_t restricted_offset;
+ struct restrictedmem_notifier notifier;
};

+static inline bool kvm_slot_can_be_private(const struct kvm_memory_slot *slot)
+{
+ return slot && (slot->flags & KVM_MEM_PRIVATE);
+}
+
static inline bool kvm_slot_dirty_track_enabled(const struct kvm_memory_slot *slot)
{
return slot->flags & KVM_MEM_LOG_DIRTY_PAGES;
@@ -1103,9 +1112,9 @@ enum kvm_mr_change {
};

int kvm_set_memory_region(struct kvm *kvm,
- const struct kvm_userspace_memory_region *mem);
+ const struct kvm_user_mem_region *mem);
int __kvm_set_memory_region(struct kvm *kvm,
- const struct kvm_userspace_memory_region *mem);
+ const struct kvm_user_mem_region *mem);
void kvm_arch_free_memslot(struct kvm *kvm, struct kvm_memory_slot *slot);
void kvm_arch_memslots_updated(struct kvm *kvm, u64 gen);
int kvm_arch_prepare_memory_region(struct kvm *kvm,
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 0d5d4419139a..f1ae45c10c94 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -103,6 +103,33 @@ struct kvm_userspace_memory_region {
__u64 userspace_addr; /* start of the userspace allocated memory */
};

+struct kvm_userspace_memory_region_ext {
+ struct kvm_userspace_memory_region region;
+ __u64 restricted_offset;
+ __u32 restricted_fd;
+ __u32 pad1;
+ __u64 pad2[14];
+};
+
+#ifdef __KERNEL__
+/*
+ * kvm_user_mem_region is a kernel-only alias of kvm_userspace_memory_region_ext
+ * that "unpacks" kvm_userspace_memory_region so that KVM can directly access
+ * all fields from the top-level "extended" region.
+ */
+struct kvm_user_mem_region {
+ __u32 slot;
+ __u32 flags;
+ __u64 guest_phys_addr;
+ __u64 memory_size;
+ __u64 userspace_addr;
+ __u64 restricted_offset;
+ __u32 restricted_fd;
+ __u32 pad1;
+ __u64 pad2[14];
+};
+#endif
+
/*
* The bit 0 ~ bit 15 of kvm_memory_region::flags are visible for userspace,
* other bits are reserved for kvm internal use which are defined in
@@ -110,6 +137,7 @@ struct kvm_userspace_memory_region {
*/
#define KVM_MEM_LOG_DIRTY_PAGES (1UL << 0)
#define KVM_MEM_READONLY (1UL << 1)
+#define KVM_MEM_PRIVATE (1UL << 2)

/* for KVM_IRQ_LINE */
struct kvm_irq_level {
@@ -1178,6 +1206,7 @@ struct kvm_ppc_resize_hpt {
#define KVM_CAP_S390_ZPCI_OP 221
#define KVM_CAP_S390_CPU_TOPOLOGY 222
#define KVM_CAP_DIRTY_LOG_RING_ACQ_REL 223
+#define KVM_CAP_PRIVATE_MEM 224

#ifdef KVM_CAP_IRQ_ROUTING

diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
index 800f9470e36b..9ff164c7e0cc 100644
--- a/virt/kvm/Kconfig
+++ b/virt/kvm/Kconfig
@@ -86,3 +86,6 @@ config KVM_XFER_TO_GUEST_WORK

config HAVE_KVM_PM_NOTIFIER
bool
+
+config HAVE_KVM_RESTRICTED_MEM
+ bool
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index e30f1b4ecfa5..8dace78a0278 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1526,7 +1526,7 @@ static void kvm_replace_memslot(struct kvm *kvm,
}
}

-static int check_memory_region_flags(const struct kvm_userspace_memory_region *mem)
+static int check_memory_region_flags(const struct kvm_user_mem_region *mem)
{
u32 valid_flags = KVM_MEM_LOG_DIRTY_PAGES;

@@ -1920,7 +1920,7 @@ static bool kvm_check_memslot_overlap(struct kvm_memslots *slots, int id,
* Must be called holding kvm->slots_lock for write.
*/
int __kvm_set_memory_region(struct kvm *kvm,
- const struct kvm_userspace_memory_region *mem)
+ const struct kvm_user_mem_region *mem)
{
struct kvm_memory_slot *old, *new;
struct kvm_memslots *slots;
@@ -2024,7 +2024,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
EXPORT_SYMBOL_GPL(__kvm_set_memory_region);

int kvm_set_memory_region(struct kvm *kvm,
- const struct kvm_userspace_memory_region *mem)
+ const struct kvm_user_mem_region *mem)
{
int r;

@@ -2036,7 +2036,7 @@ int kvm_set_memory_region(struct kvm *kvm,
EXPORT_SYMBOL_GPL(kvm_set_memory_region);

static int kvm_vm_ioctl_set_memory_region(struct kvm *kvm,
- struct kvm_userspace_memory_region *mem)
+ struct kvm_user_mem_region *mem)
{
if ((u16)mem->slot >= KVM_USER_MEM_SLOTS)
return -EINVAL;
@@ -4627,6 +4627,33 @@ static int kvm_vm_ioctl_get_stats_fd(struct kvm *kvm)
return fd;
}

+#define SANITY_CHECK_MEM_REGION_FIELD(field) \
+do { \
+ BUILD_BUG_ON(offsetof(struct kvm_user_mem_region, field) != \
+ offsetof(struct kvm_userspace_memory_region, field)); \
+ BUILD_BUG_ON(sizeof_field(struct kvm_user_mem_region, field) != \
+ sizeof_field(struct kvm_userspace_memory_region, field)); \
+} while (0)
+
+#define SANITY_CHECK_MEM_REGION_EXT_FIELD(field) \
+do { \
+ BUILD_BUG_ON(offsetof(struct kvm_user_mem_region, field) != \
+ offsetof(struct kvm_userspace_memory_region_ext, field)); \
+ BUILD_BUG_ON(sizeof_field(struct kvm_user_mem_region, field) != \
+ sizeof_field(struct kvm_userspace_memory_region_ext, field)); \
+} while (0)
+
+static void kvm_sanity_check_user_mem_region_alias(void)
+{
+ SANITY_CHECK_MEM_REGION_FIELD(slot);
+ SANITY_CHECK_MEM_REGION_FIELD(flags);
+ SANITY_CHECK_MEM_REGION_FIELD(guest_phys_addr);
+ SANITY_CHECK_MEM_REGION_FIELD(memory_size);
+ SANITY_CHECK_MEM_REGION_FIELD(userspace_addr);
+ SANITY_CHECK_MEM_REGION_EXT_FIELD(restricted_offset);
+ SANITY_CHECK_MEM_REGION_EXT_FIELD(restricted_fd);
+}
+
static long kvm_vm_ioctl(struct file *filp,
unsigned int ioctl, unsigned long arg)
{
@@ -4650,14 +4677,20 @@ static long kvm_vm_ioctl(struct file *filp,
break;
}
case KVM_SET_USER_MEMORY_REGION: {
- struct kvm_userspace_memory_region kvm_userspace_mem;
+ struct kvm_user_mem_region mem;
+ unsigned long size = sizeof(struct kvm_userspace_memory_region);
+
+ kvm_sanity_check_user_mem_region_alias();

r = -EFAULT;
- if (copy_from_user(&kvm_userspace_mem, argp,
- sizeof(kvm_userspace_mem)))
+ if (copy_from_user(&mem, argp, size))
+ goto out;
+
+ r = -EINVAL;
+ if (mem.flags & KVM_MEM_PRIVATE)
goto out;

- r = kvm_vm_ioctl_set_memory_region(kvm, &kvm_userspace_mem);
+ r = kvm_vm_ioctl_set_memory_region(kvm, &mem);
break;
}
case KVM_GET_DIRTY_LOG: {
--
2.25.1